This invention relates generally to the field of semiconductors and more specifically to methods of making high-k dielectric films used in semiconductor devices and integrated circuits.
Semiconductor devices of future generations require thin dielectric films for metal-oxide-semiconductor (MOS) gate and capacitor dielectrics. Silicon dioxide (SiO2) has been most commonly used as dielectrics in semiconductor devices due to its high integrity, low defect density and high band gap. As semiconductor device feature size is continuingly scaled down, the thickness of SiO2 layer in the integrated circuits decreases as well. Because SiO2 has a relatively low dielectric constant (k=3.9) however, such scaling soon results in a SiO2 thickness to the order of ten angstroms (Å), where charge leakage due to the quantum mechanical tunneling effect becomes significant and breakdown of the SiO2 layer may occur at even low gate voltage.
Alternative “high-k” dielectrics, materials with dielectric constants higher than that of SiO2, i.e., materials with dielectric constants of above 4, have been developed as device feature size becomes even smaller. For example, metal oxides such as Ta2O5, TiO2, Al2O3, Y2O3, ZrO2, and ferroelectric BST (barium strontium titanate) have been proposed and developed for gate dielectrics. Many of these high-k dielectric materials have sufficiently high dielectric constants and sufficient integrity at time of deposition. However, some of the high-k dielectric materials either lack chemical stability in contact with silicon substrates or lack thermal stability at temperatures typical of post-deposition processes.
It is desirable that high-k dielectric materials have a high band gap and barrier height to minimize or avoid current leakage. Band gap (Eg) is an energy gap between the highest valence band and the lowest conduction band in a solid material. Barrier height refers to the potential (voltage) barrier between a metal and a semiconductor due to the presence of a high-k dielectric. Unfortunately, most high-k dielectric materials have band gaps lower than that of SiO2 and their band gaps are inversely proportional to their dielectric constants.
Charge trapping and electron mobility degradation in semiconductor device performance are becoming serious challenges to integration of high-k dielectric materials. It is desirable that electrons in the gate channels have high mobility or less resistance to provide the device with high operating speed, enhanced performance characteristics, and low power consumption. Traditional H2O-based high-k dielectric films contain hydroxyl (OH—) impurities, which are a major source or sites for trapping charges, resulting in electron mobility degradation of high-k films.
Accordingly, further developments in high-k dielectric materials are needed to solve these and other problems of prior art dielectric materials.